01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or nonstick spray.
02 - In a large bowl, combine the diced chicken, chopped onion, thawed peas and carrots, 1 1/2 cups of cheddar cheese, sour cream, condensed cream of chicken soup, garlic powder, salt, and black pepper. Stir until evenly mixed.
03 - Transfer the chicken mixture into the prepared baking dish and spread it into an even layer.
04 - Arrange the frozen tater tots in a single layer across the entire surface of the chicken filling.
05 - Drizzle the melted butter evenly over the tater tots.
06 - Bake for 40 minutes until the tater tots begin to crisp and turn golden.
07 - Remove from the oven and sprinkle the remaining 1/2 cup of cheddar cheese evenly over the top. Return to the oven and bake for an additional 5 minutes, or until the cheese is fully melted and bubbly.
08 - Let the casserole rest for 5 minutes. Garnish with chopped fresh parsley if desired, then serve warm.